On vehicles with air suspensions, a load-dependent level control system is generally used. The amount of air in the pneumatic bellows of the air spring suspension system is thereby regulated as a function of the vehicle load so that a predetermined distance between the vehicle frame and the vehicle axle is always maintained.
A previous vehicle height control system of the present type is shown and described in U.S. Pat. No. 4,354,693. Such a prior system consists essentially of the following parts:
A distance measurement device measures the distance between the vehicle frame and the vehicle axle which varies as the vehicle is loaded or unloaded. The measure data is compared by means of an electronic evaluation circuit having a command value which is equivalent to the standard level of distance. When a deviation occurs, the standard level is re-established by pressurizing or evacuating the air springs. To fill up the air springs, the electronic system switches an air spring valve between the pressure supply source and the air springs to the open position, and the pressure supply source causes air to be conveyed into the air springs until the electronic system recognizes the standard level reported by the distance measurement device. The air pressure is then shut off and the air spring valve closed. The discharging of the air springs takes place so that the electronic system opens the air spring valve and causes a second evaluation valve to open to the atmosphere. The air springs are then discharged to the atmosphere until the electronic system closes both valves when it recognizes the standard level.
However, the systems of these previous types have the disadvantage that even when the vehicle frame descends as the result of an excessive overload, i.e., an overload of the vehicle, the pressure supply source is activated by the distance measurement device and the electronic system to re-establish the standard level. Thus, the pressure supply source continues to convey air into the bellows of the air springs, on account of the overloading of the vehicle, even though the air springs already exhibit a correspondingly excessively high air pressure. Accordingly, the air springs are filled to this extent with excess pressure and are also loaded during operation by dynamic pressure peaks, which can lead to damages to the air spring bellows. A switching malfunction caused, for example, by a defective distance measurement device and/or failure of the electronic system, is capable of causing the pressure supply source not to be shut off, which can also lead to an uncontrolled pressure increase in the air spring bellows, and thus result in the above-mentioned damage.